Metallurgical addition agent



Patented Sept. 25, 1951 METALLURGICAL ADDITION AGENT Wilbur T. Bolkcom, Hampton Township, Allegheny County, Pa., assignor to American Metallurgical Products Company, Pittsburgh, Pa., a

partnership No Drawing. Application November 30, 1949, Serial No. 130,350

2 Claims. 1

This invention relates to a metallurgical meth- 0d and material and more particularly to a method and material for furnace deoxidation and desulphurizing of metal baths.

Furnace deoxidation, or blocking as it is frequently termed in some parts of the steel industry is frequently a part of furnace practice in the metallurgy of steel and generally serves the purpose of reducing the oxygen content of the bath temporarily to a low value. This reduces or temporarily stops the reaction between the slag and the molten bath of steel so that adjustments in composition can be more easily and effectively made.

Blocking the molten bath is highly desirable particularly in the manufacture of alloy steels where variations in alloy composition cause great variations in physical properties and cost. Blocking the bath maintains the composition of the bath substantially constant over a period of time sufficient to permit samples of the metal to be removed and analyzed and to permit alteration of the composition of the bath. The block also generally increases the recovery of alloys added later to the furnace and has an effect upon the type and quantity of non-metallic inclusions produced.

The most common materials used in blocking or furnace deoxidation in basic open hearth practice are ferrosilicon, silicomanganese and spiegeleisen. These materials are, however, not entirely satisfactory for the reason that the phosphorous equilibrium between the slag and the steel is disturbed and rephosphorization of the bath may and frequently does result from their addition. In order to offset this undesirable rephosphorization it is necessary to make additions of excess burnt lime to the furnace prior to the addition of the blocking agent in order to anticipate and neutralize the rephosphorizing effect. In electric furnace practice, deoxidation and turning the slag is usually accomplished by the addition of powdered coke, sand, low-grade aluminum or calcium silicide.

Materials other than ferrosilicon, silicomanganese and spiegeleisen have from time to time been tried as blocking agents in basic open hearth furnaces with little or no practical success. For example, metallic magnesium while an excellent deoxidizer is unsuitable for furnace deoxidation because its low densityand energetic reaction permit the principal part of any addition of this material to be lost in the slag before it reaches the molten bath of metal or even before it reaches the slag. Similarly any other en-,

ergetic deoxidizer of low density which is adder to the furnace will be mostly lost before it penetrates the slag bath. Silicon carbide, on thl other extreme, has been tried as a furnace deoxidizer but has not proven successful apparently for the reason that it fails to reach a reactive temperature in sufllcient time to ente1 into reaction with the oxides of the molten batl and effect a temporary halt in the reaction between the slag and bath.

In both basic open hearth practice and basil electric furnace practice these conventional deoxidizers have little or no effect on the sulphul content of the bath. Common practice for sulphur removal in electric furnace practice is tr remove the slag from the molten bath and replace it with a synthetic slag to change the sulphur equilibrium and permit more sulphur tr migrate from the bath to the slag.- In basic ope: hearth practice sulphur removal is even more difficult and uncertain since its absolute removai from the furnace is accomplished only by the decomposition of the sulphides in the slag.

I have discovered a furnace deoxidizing anc' desulphurizing agent and a method of deoxidizing and desulphurizing metallurgical bath: by means of which rephosphorization of the bath is eliminated and other practical advantages are ained. I have found that an intimate mixture of silicon carbide particles and magnesiumfmetal particles of proper size and in proper proportion enclosed within a container which will disintegrate in the molten slag will, when added to a metallurgical bath, effectively block the bath and at the same time effect material sulphur removal The material of this invention effects a slag block as distinguished from the conventional bath block produced by the aforementioned well-known blocking agents. That is to say, the material remains in the slag where it reacts with the oxides from the metal bath at the slag-metal interface as well as with the oxides in the body of the slag and in this way produces a similar blocking or deoxidizing effect upon the molten metal.

The particles of silicon carbide in the mixture should be less than $4 in mean diameter in order to propertly cooperate with the magnesium in blocking the heat. The particles of magnesiuin should be no greater than 2" in mean diameter and preferably of the same order as the silicon carbide.

The composition of mixture should be such that premature oxidation of the magnesium is avoided and yet there is suflicient magnesium present to cause substantially complete dissemination of the silicon carbide through the metal bath and removal of the sulphur. Preferably the composition of the mixture should be essentially about 85% silicon carbide and about metallic magnesium, however, depending upon the furnace environment into which the mixture is to be added, the composition may vary between about 35% to 90% silicon carbide, about 3% to magnesium metal and the balance essentially silica which is carried along with the silicon carbide as an impurity fromv the manufacture of the silicon carbide. For example, a suitable blocking mixture for an electric furnace bath would generally-contain a smaller ratio of silicon carbide to magnesium metal than a blocking mixture for an open hearth bath.

The mixture is added to the molten bath of metal in quantities depending upon the end desired. For example, in a basic open hearth furnace where it is desired merely to block the heat for about 20 minutes while samples are removed from the bath and analyzed, then additions of about three pounds of the blocking agent of this invention per ton of molten steel are sumcient. However, if it is desired to effect material sulphur removal or as in electric furnace practice, where it is desired, not only to temporarily deoxidize the bath but also to effect a substantial sulphur reduction in the bath and at the same time create a carbidic slag, additions of as much as twenty pounds of the blocking agent of this invention per ton of molten steel may be necessary.

It is necessary that the mixture of silicon carbide and magnesium metal be added to the bath while enclosed in containers which disintegrate during the passage of the container and contents through the slag covering the bath of molten metal. Although any container which will disintegrate in the slag without leaving undesirable residues is satisfactory, it has been found that the most convenient and accordingly the preferred container is a four-ply paper bag containing pounds of the mixture. The preferred practice in using a bag of this type is to remove several of the outer layers of paper just before adding the bag to the furnace so that while the material is effectively held together when it enters the slag, the remainder of the bag is quickly disintegrated by the heat of the slag.

In adding the blocking agent to a gas-fired furnace such as an open hearth furnace, the preferred practice is to cut off the gas, add the magnesium and silicon carbide mixture inia suitable heat disintegrable container, make any manganese additions which are desired, and return the gas to the furnace. The blocking agent may be added to any furnace according to the same general practice as is used with conventional furnace deoxidizers or blocking materials. care being taken only to prevent destruction of the containers before the material is added to the bath.

As has already been pointed out the material method of the invention in addition to preventlng rephosphorization of the bath. produces certain other advantages not found in conventional blocking agents, which it is designed to replace. The blockin agent of the invention aids in the reduction of the sulphur content and assists in cleansing the steel by the formation of gaseous products which cause the slag to be stirred. The magnesium of the mixture is able to combine with a portion of the sulphur in the slag. and

the products so formed are carried out of the slag by the stirring action of the carbon monoxide gases produced by the oxidation of silicon carbide. This permits the migration of more sulphur from the molten bath to the slag to restore equilibrium. The reactions produced in the slag and at the slag-bath interface by the magnesium and silicon carbide are exothermic so that their introduction into the slag tends to increase its temperature and thereby its fluidity which per-,

mits easier migration of the undesirable oxides from the bath to the slag. Moreover, silicon carbide being by nature an acidic material thins out the basic slag which also assists in the desirable migration.

On the economic side, it must be pointed out that 300 lbs. of the material of this invention is as effective in blocking a basic open hearth heat as is 1800 lbs. of most conventional deoxidizers. The 1800 lbs. of conventional deoxidizers being merely alloys, add nothing in the way of heat to the bath but, on the contrary, cause it to chill and thicken. The material of this invention being exothermically reactive adds considerable heat to the slag and bath causing it to become thinner and effecting better migration of undesirable oxides and sulphur to the slag.

The unusual properties of the blocking agent and method of this invention appear to arise from a number of different factors. The magnesium and the silicon carbide seem to be peculiarly suitable for this combination and together produce a cooperative effect which neither one alone is capable of accomplishing. As has been pointed out above, magnesium, while it has a great affinity for oxygen and sulphur, cannot be used alone as a furnace deoxidizer with any degree of success, since it is so low in density and so reactive that it ordinarily cannot reach the molten slag in the finely divided unreacted state without being oxidized by the furnace atmosphere. Silicon carbide, on the contrary, is not sufficiently reactive for blocking purposes when added alone. Silicon carbide is a chemical compound of highly refractive nature as distinguished from conventional furnace deoxi-.

dizers, such as silicomanganese and ferrosilicon, which are alloys. This difference in chemical nature apparently affects its reactivity in the molten bath since it must, apparently, not only be dissolved. as is the case in conventional materials, but must also be disintegrated from chemical combination before it is in a reactive state. However, magnesium and silicon carbide as combined in the blocking and desulphurizing agent of this invention appear to be mutually beneficial. The silicon carbide apparently adds suflicient weight and protective covering to permit the magnesium to reach the slag-metal interi'ace and react without undue violence. The magnesium on the other hand, appears to add the necessary amount of instantaneous heat adjacent the silicon carbide particles to disseminate them in the bath and raise them to the temperature at which they will react with the oxides of the molten bath. This theory 'is consistent with the observed facts and apparently accounts for the unexpected and otherwise unexplainable results which are achieved by the blocking and desulphurizing agent of this invention. However, I

do not bind myself to this theory.

While I have illustrated and described a present preferred application of my invention, it may be otherwise embodied or practiced within the scope of the following claims.

Iclaim:

1. A metallurgical deoxidizing and desulphurizing agent comprising an intimate mixture of about 85% silicon carbide particles and about 15% metallic magnesium particles enclosed with in a heat disintegrable container.

2. A metallurgical deoxidizing and desulphurizing agent comprising an intimate mixture of about 35% to about 95% silicon carbide particles, about 3% to about 20% magnesium metal particles and the balance essentially silica enclosed within a heat disintegrable container.

WILBUR T. BOLKCOM.

REFERENCES CITED The following references are of record in the file of this patent:

0 UNITED STATES PATENTS Number Number Name Date Hunt June 22, 1897 Hunt Oct. 6, 1925 Herty Oct. 31, 1944 Brown et a1. July 6, 1948 Grifliths Aug. 10, 1948 FOREIGN PATENTS Country Date Great Britain of 1892 Great Britain of 1904 OTHER REFERENCES In Basic Open Hearth Steel Ma ing, page 492, published in 1944 bythe A. I. M. E., New York. 

1. A METALLURIGICAL DEOXIDIZING AND DESULPHURIZING AGENT COMPRISING AN INTIMATE MIXTURE OF ABOUT 85% SILICON CARBIDE PARTICLES AND ABOUT 15% METALLIC MAGNESIUM PARTICLES ENCLOSED WITHIN A HEAT DISINTEGRABLE CONTAINER. 